1. Field of Invention
The invention relates to apparatus for providing focusing information to optical imaging devices, and more specifically to cameras including focusing mechanisms that use a narrow band of wavelengths for determining the proper focus adjustment. The invention has particular utility with a source of energy, such as a visible designator or infrared source, emitting radiation in a narrow spectral band.
2. Description of the Prior Art
Camera designers are constantly providing more sophisticated features while, at the same time, simplifying camera operation for both users and manufacturers. Automatic focusing is one such capability that is both common and diverse in its applications. Many technologies are employed, including sound based systems that measure travel time; and light based systems that use image position, sharpness, triangulation, or other focus dependent parameters.
Perhaps most pertinent to the present invention are focusing systems of the type disclosed in Hisashi Goto et al. U.S. Pat. No. 5,212,375, issued May 18, 1993. Their approach uses a holographic optical element positioned between the camera objective and its image plane. Most of the light focused by the objective passes essentially undisturbed through the holographic element to form the image. Some of the light is diffracted, however, and follows different paths leading to photodetectors displaced from the image plane. The position of the diffracted light on the photodetectors changes with the position of the objective and is used to detect focus. Adjustments are controlled with information that indicates both the direction and magnitude of movement required to properly position the objective. Goto does not leave the holographic optic on the optical axis during photographing and does not propose the use of a holographic optic in an independent module separate from the main photographing lens.
Another function pertinent to the present invention is camera pointing. Pointing aids typically include frames or other indicia in the camera viewfinder that identify the center and border of the desired picture. In many cameras, the viewfinder image is formed through the same camera lens that exposes the film, providing a common frame of reference and thereby eliminating parallax. Some of the more recent pointing aids, referred to as designators, employ lasers, light emitting diodes (LEDs) or other devices that direct a narrow beam of visible light substantially parallel to the camera optics. The light beam is pointed at the subject and visibly identifies the center of the desired picture. Examples of the last mentioned approach are disclosed in Capper U.S. Pat. No. 5,189,463, issued Feb. 23, 1993, and Capper PCT Publication No. WO 93/13452, published Jul. 8, 1993. The Capper publication includes an automatic focusing mechanism that may be operated at the same time as his designator.